Solving issues and the effort it requires:
By default-we all as people have issues we need to improve on, when we are single with no children we can pretty much focus on those as we see fit. When we add a partner we now have twice as many issues to deal with. When you add additional partners you have exponentially more because-for example in a couple you have a relationship but in a group of 3 you have 4 relationships and all of the relationship issues for each + all of the 3 individual people's issues.
The ability (and commonly abused one at that) for partners to blame each other for their own problems instead of actively dealing with their shit (if you are alone its harder to blame someone else for your kitchen being dirty for a simple example).
More need to use a calendar to keep it all straight.
Less alone time.
"Love As Thou Wilt"